I plan to make some mags, some standard, others... yeah.
The highest I've made in-game is a Panzer Tail with 5/170/25/0, but I have a plan for the aforementioned mag above.
Yea, like SLU said.. probably 15+ for a custom mag and the mag cell. I also dont think max power mags are desired by most people so you would likely take a loss if you ever tried to resell it.